> The approach of A/B/C independently generating route to X only based on > local MAC learning works even if physical link failure detection is not an > option. And in most practical cases, X will have enough active flows that it > will > cause local learning on A/B/C to hopefully happen well before a failure of > connection to A happens. In the event that this is not the case, current > scheme takes care of it via ESI route.
Then -- again, aliasing is covering a corner case, and I'm not certain of the value of this mechanism in the main draft. > There are other problems with the approach that you have cited - for > instance, if the advertisement of route to X from A triggered B and C to > generate route to X as well, what happens if X goes away? Phy-layer will not > help A/B/C in the detection of X going away. A will keep its advertisement > intact (even after X has aged out locally) because it is seeing route to X > from > B/C => much like B/C created the route to X based on A's advertisement > initially. Similarly B/C will keep their routes intact because A's route is > lingering around. There will be a circular dependency created and route to X > will never get withdrawn. Not to mention, that even if this can be fixed with > extra state and extra bits in the advertisement from B/C (which may > eventually make the scheme even complex), advertisement of a route > based on another route and keeping track of this dependency in the same > route table will require a extra BGP machinery that does not exist today. Agreed -- Which is why this needs some brainstorming, rather than just adding something into BGP that is fragile in the first place. Another option might be -- - B sees A's advertisement with a MAC address it doesn't have in its local table, or hasn't learned - B initiates an ARP or some other packet towards this MAC address - If connectivity exists, normal MAC learning and advertisement now takes place As you say -- > I would think that the closer a PE is to the source of information that > causes it > to generate a route - like local MAC learning for MAC route, or LAG link > present for ESI route - the more accurate its information will be. The farther > the PE is from the source of information - like a PE depending on another PE's > route to generate its own MAC route - convergence will be slower and > complexity will be higher to keep track of dependent state. As aliasing suffers from the same criticism, it would seem, to me, that some other solution would be better overall, if we can find one. Hence my original comment that this appears not to be an elegant solution (IMHO), as it's assuming connectivity that may or may not exist, and needs more thought. :-) Russ _______________________________________________ BESS mailing list [email protected] https://www.ietf.org/mailman/listinfo/bess
